1. Field of the Invention
The present invention relates to a glass ceramic multi-layer wiring board including a circuit pattern of an excellent conductor such as copper and having a low dielectric constant and a process for making a glass ceramic green sheet to be used in the multi-layer wiring board.
2. Description of the Related Art
To treat a large quantity of information at a high speed the frequency of signal is increased and often an optical communication means is utilized.
In a multi-layer wiring or circuit board having an electric circuit for treating such high speed signals, the retardation time .tau. of electric signals must be as short as possible and cross-talk between the conductor lines must be as low as possible, and accordingly, the electric constant of the board must be small, as shown in the following formula (1): EQU .tau.=.epsilon..sup.1/2 /C . . . (1)
where .epsilon. is a dielectric constant of the board and C is a velocity of the light. Also, to lower transmission loss, the formation of an electric circuit on and in the board by a pattern of a metal material having a low electric resistance is necessary.
The inventors et al., proposed the use of a glass ceramic as a material of a board, and copper as the material of an electric circuit. Green sheets containing a ceramic such as alumina (Al.sub.2 O.sub.3,.epsilon.=10) and mullite (3Al.sub.2 O.sub.3. 2SiO.sub.2, .epsilon.=7) as a main component require a firing temperature of more than 1000.degree. C., but glass ceramic green sheets containing such ceramic powders and a glass such as borosilicate glass powder as main components can be fired at a temperature lower than 1000.degree. C., which allows copper having a melting point of 1084.degree. C. to be used as a conductor material of a circuit pattern.
Nevertheless, such glass ceramic boards have a dielectric constant of 4 to 6, which is not low enough to allow a shortening of the information retardation time.
The present invention relates to a multi-layer circuit board of a glass ceramic containing hollow or porous silica glass spheres dispersed therein. Such boards containing hollow or porous spheres dispersed therein are known, as described below.
Japanese Unexamined Patent Publication (Kokai) No. 59-111345 discloses a circuit board of a ceramic containing hollow spheres dispersed therein, and inorganic particles such as hollow alumina spheres are used as the hollow spheres.
Japanese Unexamined Patent Publication (Kokai) No. 62-206861 disclosss a ceramic multi-layer board having an insulating layer in which hollow ceramics particles are dispersed in a ceramic matrix and which has a dielectric constant of less than 5 and a bending strength of more than 5 kg/mm.sup.2. The hollow ceramic particles used are hollow silica spheres and the ceramic matrix used is originally amorphous and forms crystals in the matrix when subjected to a heat treatment, i.e., becomes a crystallized glass, to thus improve the strength of the board.
Japanese Unexamined Patent Publication (Kokai) No. 62-287658 discloses a multi-layer circuit board having alternately laminated ceramic layers and wiring layers, in which the ceramic layers comprise a glass having a softening point lower than the melting point of the wiring layer and hollow silica particles having a diameter of less than 100 .mu.m dispersed in the glass matrix.
Japanese Unexamined Patent Publication (Kokai) No. 2-83995 discloses a multi-layer circuit board having wiring layers alternately laminated with insulating layers therebetween, in which the insulating layers have two regions having different porosities and the region having a larger porosity is in contact with the wiring layer.
Japanese Unexamined Examined Patent Publication (Kokai) No. 63-358 teaches a low dielectric constant material in the form of a thick film comprising an approximately uniform mixture of hollow glass spheres with a glass ceramic. The hollow glass spheres, glass ceramic and an organic vehicle are mixed and coated on a support layer of, for example, alumina having a metal pattern formed thereon, and then dried to obtain an insulating board.
As shown above, many concepts of using hollow spheres in an insulating matrix have been disclosed, but none of them discloses a countermeasure to a crystallization of silica spheres, to form cristobalite causing a rapid increase of the thermal expansion coefficient, or a countermeasure to a formation of pores in the surface of the silica spheres by a heat treatment. The above publications, however, accelerate the crystallization of the silica spheres, to thus increase the strength of a board.
A ceramic multi-layer board composing an electric circuit required for a high speed transmission of signals, as in a supercomputer, must satisfy the following conditions:
(1) A metal material having a low electric resistance such as copper is used as the wiring or conductor material.
(2) The dielectric constant is as low as possible.
(3) The thermal expansion coefficient of the board is close to that o a silicon semiconductor.
The glass ceramic satisfies the conditions (1) and (3), but does not satisfy the condition (2). Namely, the glass ceramic has a firing temperature of less than 1000.degree. C., thereby allowing a wiring pattern made of copper to be obtained. The thermal expansion coefficient is brought close to the 3.5.times.10.sup.-6 /.degree. C. of silicon by appropriately selecting the glass as the starting material, for example, borosilicate, lead borosilicate or aluminoborosilicate. Nevertheless, the dielectric constant is disadvantageously as high as 4 to 6.
Thus, the purpose of the present invention is to provide a multi-layer board satisfying the above conditions (1) to (3), wherein the dielectric constant is not higher than 4.